In 1980 we described a new form of viral hepatitis caused by a previously unrecognized hepatitis virus with epidemiologic characteristics of HAV (i.e., the virus is fecal-orally transmitted, produces water-borne epidemics and is probably responsible for a significant proportion of endemic hepatitis) but not related to HAV. The disease has been called epidemic or enterically transmitted non-A, non-B hepatitis (ENANB). Such hepatitis is both epidemic and endemic throughout the Indian subcontinent, central Asia, parts of the Middle East and northern Africa, with probable extension to West Africa. The putative etiologic agent, a 27-30nm nonenveloped virus, was identified in a volunteer study in 1983 and similar virus-like particles have been detected in feces of cases in a number of other outbreaks. The small quantity of such virus-like particles available for study has limited progress The objectives of this study are to identify and characterize the etiologic agent of ENANB hepatitis and to develop useful assays for diagnosis of infection and seroepidemiologic studies. A longer term objective is the development of passive and active immunoprophylaxis for this important human pathogen. The identification of an acute phase stool sample that contains marginally sufficient virus-like particles for use as an antigen in immune electronmicroscopic studies of ENANB has permitted limited cross-serologic studies of paired sera obtained from epidemic and endemic disease in regions where the virus is prevalent. From these studies we conclude that ENANB hepatitis occurring throughout India, in Pakistan, southern Russia and Algeria is all caused by serologically related or identical viruses. Chimpanzees inoculated with stools containing virus-like particles from the outbreaks developed hepatitis and seroconverted, thus proving transmissibility to a non-human primate species.